(1) Field of the Invention
The present invention relates to a photomask used in exposing photoresist applied to a semiconductor substrate to ultraviolet rays.
(2) Description of the Related Art
In recent years, with the development of semiconductor technology, high-performance and low-price semiconductor devices have been mass-produced. Correspondingly, performance improvement and price reduction in electronic equipment using semiconductor devices, such as general purpose computers, personal computers, personal telephones and television game devices, have been greatly advanced. In order to distinguish one from others in terms of performance in these products, the products each provided with several kinds of unique functions have competitively been made available to customers. In this case, it is difficult to achieve a unique function using only general purpose semiconductor devices. So, in the fabrication of the semiconductor devices, custom LSIs with unique functions are required. The custom LSIs include a semi-custom LSI called a "gate array" which is fabricated up to an intermediate step of process in a general way and modified in various ways in a wiring step. The gate array is inexpensive, and can be fabricated in a relatively short time because it has only to be uniquely designed in the wiring step. For this reason, in recent years, the gate arrays have been mass-produced.
Formation of wirings for the gate array is effected in such a way that, after a wiring pattern is designed, the wiring pattern made of metal such as chromium (Cr) formed on a glass substrate, which is called a mask or a reticle, is transferred on to a semiconductor substrate by an exposure device called a stepper. FIGS. 1A through 1D are longitudinal sectional views for explaining a conventional reticle and a method of making the reticle. In making the reticle, first, an electron beam resist is applied to a glass substrate 1 covered with a chromium (Cr) film 2. In accordance with design data for a reticle pattern, an electron beam exposure system writes the pattern using electron beams 4 as shown in FIG. 1A. The electron beam resist 3 is developed as shown in FIG. 1B and, then, using the resultant electron beam resist 3 as a mask, the chromium (Cr) film 2 is etched away as shown in FIG. 1C. Finally, the electron beam resist 3 is removed to thereby complete a reticle as shown in FIG. 1D. FIG. 2A is a partial plan view of the completed reticle, and FIG. 2B is a sectional view taken along the line A-A' in FIG. 2A. The wiring pattern is formed by the chromium (Cr) film 2.
The above prior art has the following problems to be solved. In the prior art, in order to make a reticle, first, it is necessary to convert design data for a reticle pattern into a data format which is peculiar to the electron beam exposure system used. This requires a very long time for conversion and a great expense for using a computer. Further, in the pattern writing using the electron beam exposure system, the writing time increases substantially in proportion to the increase in the number or area of reticle patterns. Thus, with the advancement in the large-scaling of LSIs in recent years, the writing time is rapidly increasing and many hours, for example, 24 hours are spent only for the data conversion and the pattern writing. In this way, using the conventional reticle presents a problem in that it takes a long time and is costly to fabricate a custom LSI which is expected to be or should be supplied in a short time and at low cost. These are problems to be solved by the invention, in the conventional photomasks for the semiconductor integrated circuit devices.